In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted. The three key parameters of a periodic waveform are its amplitude, its phase and its frequency. Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used.
The purpose of modulation is to convey a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband.
FIG. 1 shows the waveforms resulting from Amplitude Modulation (AM) and Frequency Modulation (FM) of a carrier where 101 is the low frequency modulating signal, 102 is the amplitude modulated carrier and 103 shows the frequency modulated carrier.
Most FM broadcast stations transmit stereo signals, however it is important that the stereo broadcasts be compatible with mono receivers. For this reason as shown on FIG. 2, the left (L) and right (R) audio channels are algebraically encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels through the single loudspeaker. A stereo receiver will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel.
The (L+R) main channel signal is transmitted as baseband audio 201 in the range of 30 Hz to 15 kHz. The (L−R) signal is modulated onto a 38 kHz double-sideband suppressed carrier (DSBSC) signal 202 and 203 occupying the baseband range of 23 to 53 kHz.
A 19 kHz pilot tone 204, at exactly half the 38 kHz sub-carrier frequency and with a precise phase relationship to it is also generated. This is transmitted at 8-10% of overall modulation level and used by the receiver to regenerate the 38 kHz sub-carrier with the correct phase.
The final multiplex (Mpx) signal from the stereo generator contains the Main Channel (L+R), the pilot tone, and the sub-channel (L−R). This composite signal, along with any other sub-carriers, modulates the FM transmitter.
Content transmitted by FM stations may have varying frequency deviation from the nominal carrier frequency depending on the transmission bandwidth the station uses, and the instantaneous bandwidth of the message signal that is modulating the FM carrier. In order to minimize distortion, the bandwidth employed must be wide enough to accommodate maximum frequency deviation allowed by the particular station.
An FM receiver is unaware of the frequency deviation and has to make an estimate of it, or some reasonable time-average of it, and accordingly set the input filter's bandwidth. If the input filter's bandwidth is narrower than the required bandwidth, it will lead to distortion because necessary portions of the message spectrum have been clipped off. If the input's filter bandwidth is much wider than the required bandwidth, it may not harm performance if the RF signal level is strong. However, if the RF signal level is weak, it will contribute to additional noise and thus degrade performance. By setting the input filter's bandwidth to just the necessary bandwidth, the receiver will maximize performance both in terms of Signal-to-Noise Ratio (SNR) and Signal-to-Noise-And-Distortion (SINAD). Thus, receivers try to be frequency deviation “followers”.
In the receiver, a parameter called Modulation is a measure of the frequency deviation, using which a radio receiver estimates what bandwidth of the input signal must be used for decoding and recovering the message signal.
Estimation of modulation is a difficult problem. The estimation is particularly hard when the RF signal level is weak, and when noise corrupts the signal. Typically, modulation can be measured by looking at the peaks of the recovered (modulating) signal. However, under weak signal conditions, signal peaks get blanketed by noise peaks, so the peak-based approach overestimates the frequency deviation, thereby leading to a wider input filter than necessary and thus hurting SNR and SINAD.